Cascaded Modulator System and Method for Qkd

1. An optical modulator system for modulating an input optical signal, comprising: first and second optical modulators optically coupled in series so as to sequentially operate on the input optical signal without splitting the input optical signal prior to said first modulator; first and second voltage adjusters respectively electrically coupled to the first and second optical modulators; a parallel shift register electrically coupled to the first and second voltage adjusters so as to provide respective binary voltages to the voltage adjusters; and wherein the voltage adjusters act on the binary voltages to create first and second voltages corresponding to desired phase modulation values for the first and second optical modulators so as to provide four possible net modulations.

2. The optical modulator system of claim 1 , further including a random number generator electrically coupled to the parallel shift register so as to randomize the binary voltage output of the parallel shift register so that the net modulation is randomly selected from the four possible net modulations.

3. The modulator system of claim 1 , including an electrical delay element between the second voltage adjuster and the second optical modulator, wherein the electrical delay element provides an electrical delay equal to an optical delay between the first and second optical modulators.

4. The optical modulator system of claim 1 , including an input optical fiber optically coupled to the first optical modulator, and an output optical fiber optically coupled to the second optical modulator.

5. The optical modulator system of claim 1 , wherein the four possible net modulations are either (a)±3π/4 and ±π/4 or (b)±π/2 and ±π/4.

6. The optical modulator system of claim 1 , wherein the first and second optical modulators are optically coupled by a section of optical fiber.

7. The optical modulator system of claim 2 , wherein the electrical delay element is configured so that the electrical delay is adjustable.

8. An optical modulator system for modulating an input optical signal, comprising: first and second optical modulators optically coupled in series and having an optical delay therebetween so as to sequentially operate on the input optical signal without splitting the input optical signal prior to said first modulator; first and second voltage adjusters respectively electrically coupled to the first and second optical modulators; an electrical delay element between the second voltage adjuster and the second optical modulator and configured to provide an electrical delay that compensates for the optical delay between the first and second optical modulators; a parallel shift register electrically coupled to the first and second voltage adjusters so as to provide respective first and second binary voltages to the first and second voltage adjusters; and wherein the voltage adjusters act on the first and second binary voltages to create first and second phase modulation voltages corresponding to desired first and second phase modulation values for the first and second optical modulators so that the optical modulator system can provide four possible net phase modulations.

9. The optical modulator system of claim 8 , wherein the electrical delay element is configured so that the electrical delay is adjustable.

10. The optical modulator system of claim 8 , further including a random number generator electrically coupled to the parallel shift register so as to randomize the binary voltage output of the parallel shift register so that the net modulation is randomly selected from the four possible net modulations.

11. The optical modulator system of claim 8 , further comprising an input optical fiber optically coupled to the first optical modulator, and an output optical fiber optically coupled to the second optical modulator.

12. The optical modulator system of claim 8 , wherein the first and second optical modulators are optically coupled by a section of optical fiber.